1. Preheat the oven to 350°F (The rack should be set in the lower third of the oven.) Bring 1 quart water to a boil. Wrap a piece of foil around the bottom and sides of a springform pan. (This prevents water from leaking in.)

2. Puree the cottage cheese in the food processor, scraping down the sides several times. This will take 2 to 3 minutes. Add the cream cheese and puree until smooth. Add the sugar and puree. Add the eggs, egg whites, lemon juice, lemon zest, vanilla, and salt and puree. Pour the mixture into the prepared pan. Tap the pan a few times on the work counter to knock out any bubbles.

3. Set the pan in a roasting pan in the oven. Add 1 inch boiling water to the roasting pan and bake the cheesecake for 30 minutes.

4. Meanwhile, in a large mixing bowl, whisk together the sour cream and sugar. Spoon this mixture on top of the cheesecake. Continue baking the cheesecake until set, 10 to 20 minutes longer. To test for doneness, gently poke the side — when the top no longer jiggles, the cheesecake is cooked. Another test: an inserted skewer will come out clean when the cheesecake is cooked. Do not overcook, or the cheesecake will become watery.

5. Transfer the cheesecake to a cake rack to cool to room temperature, then refrigerate until cold. To serve, run the tip of a small knife around the inside of the pan. Unfasten the sides.